Zoom lens and image pickup apparatus

ABSTRACT

The present invention provides a zoom lens suitable for an image pickup system using a solid-state image pickup element. In detail, the zoom lens disclosed in the present invention includes three lens units having negative, positive, and positive optical power in order from an object side to an image side, in which zooming is performed by moving each lens unit in the direction of an optical axis. The first lens unit has at least one positive lens and at least one negative lens. The second lens unit has at least one positive lens and at least one negative lens, in which a plastic lens of positive optical power is disposed on the side closest to the object.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a zoom lens suitable for still cameras,video cameras, or digital still cameras, and relates to an image pickupapparatus using the lens.

[0003] 2. Description of Related Art

[0004] Recently, with the trend toward making an image pickup apparatus(camera), such as a video camera or a digital still camera that uses asolid-state image pickup element, compact, or toward making theapparatus highly functional, demands have been made for a zoom lens inwhich the entirety of a lens system is compact, is superior inperformance, and is capable of being easily manufactured.

[0005] In this type of camera, various optical members, such as alow-pass filter and a color correction filter, are disposed between arearmost lens part and an image pickup element, and, accordingly, a lenssystem with a relatively long back focus is required for an opticalsystem used for it. Further, in a color camera that employs an imagepickup element used for color images, an optical system used for it isdesired to be superior in telecentric characteristics on an image side,in order to avoid color shading.

[0006] Conventionally, a two-unit zoom lens of a so-called short zoomtype which consists of two lens units, i.e., a first lens unit havingnegative optical power (the optical power is the reciprocal of a focallength) and a second lens unit having positive optical power and inwhich the variation of magnification is performed while changing a lensinterval between the two units is known as a zoom lens designed to becompact. In this short zoom type optical system, the variation ofmagnification is performed by moving the second lens unit of positiveoptical power, and image-point position compensation that depends on thevariation of magnification is performed by moving the first lens unit ofnegative optical power.

[0007] Further, various optical systems that use a large number ofplastic lenses are proposed in order to easily manufacture a producthaving a simple structure. For example, Japanese Laid-Open No. H3-15513proposes an optical system applied to the short zoom type two-unit zoomlens.

[0008] Generally, the short zoom type two-unit zoom lens has a long backfocus required for an optical system that uses the above-mentionedsolid-state image pickup element, and has difficulty in keepingtelecentric characteristics superior.

[0009] In order to lengthen the back focus and improve the telecentriccharacteristics, a three-unit zoom lens system in which a third lensunit having positive optical power is disposed on the image plane sideof the short zoom type two-unit zoom lens has been proposed, forexample, in Japanese Laid-Open No. S63-135913 (corresponding to U.S.Pat. No. 4,838,666), Japanese Laid-Open No. H7-261083, and JapaneseLaid-Open No. 2000-111798 (corresponding to U.S. Pat. No. 6,308,011).

[0010] In the three-unit zoom lens proposed in these publications, thenumber of lenses making up each lens unit is relatively large, and theentire length of a lens system is long. In addition, there was atendency towards difficulties in manufacturing, as a glass-madeaspherical lens is used.

[0011] Like the short zoom type two-unit zoom lens, a three-unit zoomlens that consists of lens units of negative-positive-positive opticalpower that use plastic lenses is disclosed, for example, in JapaneseLaid-Open No. H5-323190 (corresponding to U.S. Pat. No. 5,357,374) andJapanese Laid-Open No. H9-21950.

[0012] In Japanese Laid-Open No. H5-323190, each lens unit is made up ofsingle plastic lens. Although excellent from a manufacturing viewpoint,improvements must be made to its optical performance from the viewpointof a zoom lens to be used for a high-pixel solid-state image pickupelement of the present day.

[0013] In Japanese Laid-Open No. H9-21950, the first and second lensunits are moved while the third lens unit is being fixed when zooming,and at least two plastic lenses are used for the first and second lensunits.

[0014] The zoom lens disclosed in these publications has the followingdisadvantages.

[0015] Since the third lens unit is fixed when zooming, variation ofmagnification needs to be performed only by the second lens unit, makingit difficult to perform aberration correction.

[0016] The number of plastic lenses is half or less than the number ofall lenses, and there is room for improvement from a manufacturingviewpoint.

[0017] Since the nearest-to-object lens of the second lens unit in whichan axial light ray is most distant from an optical axis is a glass-madelens, it is difficult to correct spherical aberrations and comaticaberrations.

SUMMARY OF THE INVENTION

[0018] The present invention has been made in consideration of theseconventional examples, and it is an object of the present invention toprovide a zoom lens that is suitable for a photographic system using asolid-state image pickup element, small in the number of lenses,compact, and has excellent optical performance.

[0019] In order to achieve the above-mentioned object, the zoom lens ofthe present invention includes three lens units of negative, positive,and positive optical powers in order from an object side to an imageside, in which zooming is performed by moving each lens unit in thedirection of an optical axis. A first lens unit has at least onepositive lens and at least one negative lens. A second lens unit has atleast one positive lens and at least one negative lens, and a plasticlens of positive optical power is disposed on a closest-to-object side.

[0020] A more concrete form of the present invention will becomeapparent from embodiments described later.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a sectional view of a zoom lens of Embodiment 1.

[0022]FIG. 2 is a view showing the aberration at a wide-angle end of thezoom lens of Embodiment 1.

[0023]FIG. 3 is a view showing the aberration at a middle zoom positionof the zoom lens of Embodiment 1.

[0024]FIG. 4 is a view showing the aberration at a telephoto end of thezoom lens of Embodiment 1.

[0025]FIG. 5 is a sectional view of a zoom lens of Embodiment 2.

[0026]FIG. 6 is a view showing the aberration at a wide-angle end of thezoom lens of Embodiment 2.

[0027]FIG. 7 is a view showing the aberration at a middle zoom positionof the zoom lens of Embodiment 2.

[0028]FIG. 8 is a view showing the aberration at a telephoto end of thezoom lens of Embodiment 2.

[0029]FIG. 9 is a sectional view of a zoom lens of Embodiment 3.

[0030]FIG. 10 is a view showing the aberration at a wide-angle end ofthe zoom lens of Embodiment 3.

[0031]FIG. 11 is a view showing the aberration at a middle zoom positionof the zoom lens of Embodiment 3.

[0032]FIG. 12 is a view showing the aberration at a telephoto end of thezoom lens of Embodiment 3.

[0033] FIGS. 13(A) and 13(B) are a schematic views of a main part of adigital still camera.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Hereinafter, preferred embodiments of the invention will bedescribed in detail with reference to the drawings.

[0035]FIG. 1 is a sectional view of a zoom lens of Embodiment 1described later. FIG. 2 through FIG. 4 are views showing the aberrationsat a wide-angle end, at a middle zoom position, and at a telephoto end,respectively, of the zoom lens of Embodiment 1.

[0036]FIG. 5 is a sectional view of a zoom lens of Embodiment 2described later. FIG. 6 through FIG. 8 are views showing the aberrationsat a wide-angle end, at a middle zoom position, and at a telephoto end,respectively, of the zoom lens of Embodiment 2.

[0037]FIG. 9 is a sectional view of a zoom lens of Embodiment 3described later. FIG. 10 through FIG. 12 are views showing theaberrations at a wide-angle end, at a middle zoom position, and at atelephoto end, respectively, of the zoom lens of Embodiment 3.

[0038] In each lens sectional view, L1 designates a first lens unithaving negative optical power, L2 designates a second lens unit havingpositive optical power, L3 designates a third lens unit having positiveoptical power, SP designates an aperture stop, and IP designates animage plane. G designates a glass block, such as a filter or a colorseparation prism.

[0039] The zoom lens of each embodiment includes three lens units, i.e.,the first lens unit L1 of negative optical power, the second lens unitL2 of positive optical power, and the third lens unit L3 of positiveoptical power in order from an object side, and, during zooming from thewide-angle end to the telephoto end, the first lens unit L1reciprocatively moves in a convex locus toward the image side, thesecond lens unit L2 moves to the object side, and the third lens unit L3moves to the object side or to the image plane side.

[0040] Variation of magnification is mainly performed by moving thesecond lens unit L2. A variation of an image point caused by thevariation of magnification is corrected by the reciprocative movement ofthe first lens unit L1 and the movement of the third lens unit L3.

[0041] The third lens unit L3 shares an increase in the optical power ofthe zoom lens resulting from a size reduction of an image pickupelement, and, by reducing the optical power of a short zoom system madeup of the first and second lens units L1 and L2, aberrations areprevented from occurring especially in each lens making up the firstlens unit L1, thereby achieving a superior optical performance.

[0042] Further, telecentric image formation on the image side necessaryfor an image pickup apparatus (optical apparatus) that uses asolid-state image pickup element as an image pickup element is achievedby allowing the third lens unit L3 to have a role of a field lens.

[0043] Further, the effective outer diameter of lenses that constitutethe first lens unit L1 is prevented from increasing by disposing theaperture stop SP on the closest-to-object side of the second lens unitL2 and by diminishing the distance between an entrance pupil and thefirst lens unit L1 on the wide angle side. Further, without increasingthe number of constituent lenses, an excellent optical performance isobtained by canceling off-axis aberrations by means of the first andthird lens units L1 and L3 with the aperture stop disposed on the objectside of the second lens unit L2 therebetween.

[0044] Next, the lens structure of each embodiment will be described.

[0045] Embodiment 1 of FIG. 1 shows a zoom lens in which the variablemagnification ratio is 2 and in which the aperture ratio is about 2.6 to4.0.

[0046] In Embodiment 1, the first lens unit L1 of negative optical powerconsists of two lenses of a meniscus-like negative lens 11 in which theconcave surface is directed to the image plane side and a meniscus-likepositive lens 12 in which the convex surface is directed to the objectside in order from the object side.

[0047] The second lens unit L2 of positive optical power consists offour lenses of a meniscus-like positive lens 21 in which the concavesurface is directed to the image plane side, a meniscus-like negativelens 22 in which the convex surface is directed to the object side, ameniscus-like negative lens 23 in which the convex surface is directedto the object side, and a positive lens 24 both lens surfaces of whichare convex in order from the object side, in which the positive lens 21and the negative lens 22 are cemented to each other, and the negativelens 23 and the positive lens 24 are cemented to each other.

[0048] The third lens unit L3 of positive optical power consists of ameniscus-like positive lens 31 in which the convex surface is directedto the object side.

[0049] In this embodiment, the material of the positive lens 21, thenegative lens 22, and the negative lens 23 of the second lens unit andthe material of the positive lens 31 of the third lens unit are plastic(resin), and, in all of the lenses having a seven-lens structure, fourlenses are plastic. The material of the positive lens is acrylic, andthat of the negative lens is polycarbonate.

[0050] Embodiment 2 of FIG. 5 shows a zoom lens in which the variablemagnification ratio is 2.5 and in which the aperture ratio is about 2.8to 4.0.

[0051] In Embodiment 2, the first lens unit L1 of negative optical powerconsists of three lenses of a meniscus-like negative lens 11 in whichthe concave surface is directed to the image plane side, a meniscus-likenegative lens 12 in which the concave surface is likewise directed tothe image plane side, and a meniscus-like positive lens 13 in which theconvex surface is directed to the object side in order from the objectside.

[0052] The second lens unit L2 of positive optical power consists ofthree lenses of a positive lens 21 both lens surfaces of which areconvex, a negative lens 22 both lens surfaces of which are concave, anda positive lens 23 both lens surfaces of which are convex in order fromthe object side.

[0053] The third lens unit L3 of positive optical power consists of apositive lens 31 both lens surfaces of which are convex.

[0054] In this embodiment, the material of the negative lens 11 of thefirst lens unit L1, the material of the positive lens 21 and thenegative lens 22 of the second lens unit L2, and the material of thepositive lens 31 of the third lens unit are plastic, and, in all of thelenses having a seven-lens structure, four lenses are plastic. Thematerial of the negative lens 11, the positive lens 21, and the positivelens 31 is acrylic, and that of the negative lens 22 is polycarbonate.

[0055] Embodiment 3 of FIG. 9 shows a zoom lens in which the variablemagnification ratio is 2 and in which the aperture ratio is about 2.8 to4.0.

[0056] In this embodiment, during zooming from the wide-angle end to thetelephoto end, the first lens unit and reciprocatively moves in a convexlocus toward the image side, the second lens unit moves to the objectside, and the third lens unit moves in a convex locus toward the imageside.

[0057] In Embodiment 3, the first lens unit L1 of negative optical powerconsists of three lenses of a meniscus-like negative lens 11 in whichthe concave surface is directed to the image plane side, a meniscus-likenegative lens 12 in which the concave surface is likewise directed tothe image plane side, and a positive lens 13 both lens surfaces of whichare convex in order from the object side.

[0058] The second lens unit L2 of positive optical power consists offour lenses of a positive lens 21 both lens surfaces of which areconvex, a negative lens 22 both lens surfaces of which are concave, ameniscus-like negative lens 23 in which the convex surface is directedto the object side, and a positive lens 24 both lens surfaces of whichare convex in order from the object side, and the negative lens 23 andthe positive lens 24 are cemented to each other.

[0059] The third lens unit L3 of positive optical power consists of apositive lens 31 both lens surfaces of which are convex.

[0060] In this embodiment, the material of the negative lens 11 and thepositive lens 13 of the first lens unit, the material of the positivelens 21, the negative lens 22, and the negative lens 23 of the secondlens unit, and the material of the positive lens 31 of the third lensunit are plastic, and, in all of the lenses having an eight-lensstructure, six lenses are plastic. The material of the negative lens 11,the positive lens 21, and the positive lens 31 is acrylic, and thematerial of the positive lens 13, the negative lens 22, and the negativelens 23 is polycarbonate.

[0061] As described above, by forming a lens structure in which eachlens unit is disposed to have a desired optical power and to performaberration correction, the lens system can be made compact whilemaintaining a superior optical performance.

[0062] Next, features common to each embodiment mentioned above will bedescribed.

[0063] The first lens unit L1 has a role which allows an off-axisprincipal ray to form an image of the pupil on the center of theaperture stop SP, and, since the off-axis principal ray is large in theamount of refraction especially on the wide angle side, off-axisaberrations, especially astigmatism and distortion, easily occur.

[0064] Therefore, like a wide-angle lens used in most cases, a two-lensstructure having, in order from the object side, a negative lens and apositive lens, or a three-lens structure having a negative lens, anegative lens, and a positive lens is employed in which the diameter ofthe lens on the closest-to-object side is prevented from increasing.

[0065] In addition, astigmatism and distortion are corrected in a wellbalanced manner by, if necessary, forming the lens surface on the imageside of the meniscus-like negative lens 11 into an aspherical surface inwhich the negative optical power becomes weak on the periphery.

[0066] Herein, the negative lens 11 has the largest lens diameter, andgreater advantages in manufacture and in weight can be obtained bymanufacturing the negative lens 11 with plastic material as inEmbodiment 2 and in Embodiment 3 than a case in which this lens ismanufactured through grinding or molding with glass material.

[0067] Each lens that constitutes the first lens unit L1 is formed in ashape similar to a concentric spherical surface that centers theintersection of the stop SP and the optical axis, in order to preventthe occurrence of off-axis aberrations caused by the refraction of anoff-axis principal ray.

[0068] In the second lens unit L2, a positive lens 21 in which thestrong convex surface is directed to the object side is disposed on theclosest-to-object side in the lens unit, so that the refractive angle ofan off-axis principal ray emitted from the first lens unit L1 isreduced, and thereby various off-axis aberrations do not preferablyoccur whenever possible.

[0069] The positive lens 21 is a lens through which an axial ray travelsat the greatest height, and the lens 21 is involved chiefly incorrecting spherical aberrations and comatic aberrations.

[0070] The fact that the height at which an axial ray travels throughthe lens is the highest indicates that the height is a place where theeffect of the aspherical surface can be most brought about in lensesthat constitute the second lens unit L2. In each embodiment, the lenssurface on the object side of the positive lens 21 is shaped to beaspherical so that positive optical power becomes weak on the periphery,thus correcting the spherical aberrations and the comatic aberrationsfavorably.

[0071] Further, like the negative lens 11 of the first lens unit L1, agreater advantage in manufacture can be obtained by manufacturing thepositive lens 21 with plastic material than a case in which it ismanufactured with glass material.

[0072] Further, the negative lens 22 made of plastic is disposedadjacent to the image plane side of the positive lens 21. As a result,in the zoom lens of a three-unit structure having negative, positive,and positive optical power lens units, sensitivity to the surface-shapechange of lenses that constitute the second lens unit serving chieflyfor variation of magnification is higher than lenses of the other lensunits. Thus although the influence of the surface-shape change caused bytemperature/humidity is a matter of most concern by using plasticlenses, aberration variations are canceled by the fact that the negativelens 22 that is paired with the positive lens 21 is a plastic lens toproduce the amount of spherical aberration or comatic aberration with anopposite sign.

[0073] The third lens unit L3 consists of a positive lens 31 in whichthe convex surface is directed to the object side and serves as a fieldlens in which the image side is telecentric. The surface on the objectside of the positive lens 31 is shaped to be aspherical so that positiveoptical power becomes weak on the periphery, contributing to thecorrection of various off-axis aberrations in the entire area ofzooming.

[0074] Herein, like the negative lens 11 of the first lens unit L1 andlike the positive lens 21 of the second lens unit L2, a greateradvantage in manufacture can be obtained by manufacturing the positivelens 31 with plastic material than a case in which it is manufacturedwith glass material.

[0075] Although a superior optical performance can be obtained by movingthe first lens unit L1 to the object side, it is more desirable to movethe third lens unit L3 to the object side when taking images of ashort-range object from an infinite object by use of the zoom lens ofeach embodiment.

[0076] The reason being that it is possible to stop an increase in lensdiameter caused by focusing the first lens unit L1 disposed on theclosest-to-object side and stop an increase in the load of an actuatorcaused by moving the first lens unit L1 in which the weight is heaviest,and it becomes possible to move the first lens unit L1 and the secondlens unit L2 which are simply connected to each other, for example, witha cam during zooming, thereby achieving simplification of a mechanicalstructure and achieving an accuracy improvement.

[0077] Further, when focusing is performed by the third lens unit L3,the telephoto end that has a great focusing-movement amount can bedisposed on the image plane side by moving the third lens unit L3 to theimage side in variation of magnification from the wide-angle end to thetelephoto end, and therefore it becomes possible to minimize allmovement amounts of the third lens unit L3 needed in zooming and infocusing, thereby making the lens system compact.

[0078] Further, in each embodiment, the total number of plastic lensesin the lens system is even, and the number of positive-optical-powerplastic lenses is equal to the number of negative-optical-power plasticlenses, and therefore, like the relationship between the positive lens21 and the negative lens 22 of the second lens unit L2, aberrationvariations caused by environmental changes are canceled by allowingaberrations generated in the positive lens to have an opposite sign withrespect to aberrations generated in the negative lens while beinginfluenced by a surface-shape change caused by temperature/humidity. Inorder to obtain an excellent optical performance, the zoom lens of eachembodiment satisfies the following conditions. In the zoom lens of thepresent invention, an effect, by which the optical performance isimproved or the size of the entire lens system is reduced by satisfyingeach conditional expression, can be obtained by satisfying at least oneof the conditions.

[0079] (A-1)

[0080] The following condition is satisfied:

0.5<f2s/f2<0.9  Conditional Expression (1)

[0081] where f2s is the focal length of the positive optical power lens21 disposed on the closest-to-object side of the second lens unit L2,and f2 is the focal length of the second lens unit L2.

[0082] If the upper limit value of Conditional Expression (1) isexceeded, correction deficiencies arise in the spherical aberration,which is undesirable.

[0083] If the lower limit of Conditional Expression (1) is exceeded, adifficulty arises in correcting spherical aberrations and comaticaberrations, which is undesirable.

[0084] (A-2)

[0085] The following condition is satisfied:

0.9<n2p/n2n<1.0  Conditional Expression (2)

[0086] where n2p is the refractive index of the material of thepositive-optical-power lens 21 disposed on the closest-to-object side ofthe second lens unit L2, and n2n is the refractive index of the materialof the negative-optical-power lens 22 disposed adjacent thereto.

[0087] If the upper limit value of Conditional Expression (2) isexceeded, the Petzval sum increases in the negative direction, and adifficulty arises in correcting the curvature of field.

[0088] If the lower limit value of Conditional Expression (2) isexceeded, the Petzval sum increases in the positive direction incontrast with the above, and, likewise, a difficulty arises incorrecting the curvature of field, which is undesirable.

[0089] According to the zoom lens of each embodiment, each element isset as described above, and therefore it is possible to realize a zoomlens that is suitable especially for an image pickup system using asolid-state image pickup element, that is small in the number ofconstituent lenses, that is compact, and that has an excellent opticalperformance.

[0090] Further, since the lens made of plastic is effectively used, andsince the aspherical surface is employed, it is possible to effectivelycorrect various off-axis aberrations, especially astigmatism anddistortion, and correct spherical aberration caused by increasing theaperture ratio.

[0091] Next, Numeric Examples 1 through 3 that correspond to Embodiments1 through 3, respectively, will be shown. In each numeric example, idesignates the order of surfaces from the object side, ri designates thecurvature radius of a i+th surface, di designates a lens thickness andan air interval between the i−th surface and the i+1−th surface, ni andvi designate the refractive index and the Abbe number, respectively,with respect to the d line. f designates a focal length, Fno designatesF number, and ω designates a half angle of field.

[0092] Two surfaces on the closest-to-image side are parallel flatplates that correspond to, for example, face plates. k is conicalcoefficient, and B, C, D, E, and F are aspherical coefficients. Theaspherical shape is expressed by the following equation:

x=(h ² /R)/[1+{1−(1+k)(h/R)²}^(½) ]+Bh ⁴ +Ch ⁶ +Dh ⁸ +Eh ¹⁰ +Fh ¹²

[0093] where x is a displacement with height “h” from the optical axisbased on a facial vertex, and R is a curvature radius.

[0094] The relationship between each conditional expression and eachembodiment (numeric example) is shown in Table 1.

[0095] Numeric Example 1 f = 5.38 − 10.79 fno = F2.6 − 4.0 2ω = 62.6° −33.8° r1 = 34.243 d1 = 1.20 n1 = 1.80610 ν1 = 40.7 r2 = 3.571 d2 = 1.10r3 = 6.713 d3 = 2.00 n2 = 1.84666 ν2 = 23.9 r4 = 30.266 d4 = Variable r5= (Aperture stop) d5 = 0.80 r6 = 3.324 d6 = 1.80 n3 = 1.49171 ν3 = 57.4r7 = 12.692 d7 = 0.50 n4 = 1.58306 ν4 = 30.2 r8 = 3.419 d8 = 0.68 r9 =21.534 d9 = 0.60 n5 = 1.58306 ν5 = 30.2 r10 = 6.695 d10 = 1.60 n6 =1.69680 ν6 = 55.5 r11 = −12.444 d11 = Variable r12 = 9.015 d12 = 1.50 n7= 1.49171 ν7 = 57.4 r13 = 800.264 d13 = Variable r14 = ∞ d14 = 2.80 n8 =1.51633 ν8 = 64.1 r15 = ∞ Focal length variable interval 5.38 7.64 10.79d4 7.66 4.70 2.07 d11 1.90 7.53 12.77 d13 3.98 2.70 1.82

[0096] Aspherical coefficient Surface number r K B C D E F 2 3.57083D+00−1.00599D+00 6.78632D−04 1.10721D−05 −1.48659D−06 1.82326D−07−7.74158D−09 6 3.32379D+00 −1.08624D+00 2.10302D−03 4.97777D−053.10005D−06 12 9.01539D+00   0.00000D+00 −5.19269D−04 9.18414D−05−7.03831D−06 2.06886D−07

[0097] f = 4.00 − 10.00 fno = F2.8 − 4.0 2ω = 78.6° − 36.2° r1 = 20.882d1 = 1.00 n1 = 1.49171 ν1 = 57.4 r2 = 5.405 d2 = 2.52 r3 = 51.498 d3 =0.60 n2 = 1.80400 ν2 = 46.6 r4 = 7.206 d4 = 1.04 r5 = 9.932 d5 = 2.00 n3= 1.84666 ν3 = 23.8 r6 = 46.317 d6 = Variable r7 = (Aperture stop) d7 =0.00 r8 = 4.712 d8 = 1.80 n4 = 1.49171 ν4 = 57.4 r9 = −10.725 d9 = 0.32r10 = −8.830 d10 = 1.40 n5 = 1.58306 ν5 = 30.2 r11 = 5.043 d11 = 0.43r12 = 8.709 d12 = 1.60 n6 = 1.69680 ν6 = 55.5 r13 = −13.026 d13 =Variable r14 = 19.910 d14 = 1.50 n7 = 1.49171 ν7 = 57.4 r15 = −302.095d15 = Variable r16 = ∞ d16 = 3.40 n8 = 1.51633 ν8 = 64.1 Focal lengthvariable interval  4.00 7.09 10.00 d6 13.14 4.71 1.55 d13 2.21 5.05 8.03d15 3.36 4.36 5.21

[0098] Aspherical coefficient Surface number r K B C D E F G 25.40547D+00 0.00000D+00 1.79936D−02 −5.61701D−04 1.61551D−05−2.24582D−06 7.63002D−08 −2.1244D−09 8 4.71230D+00 −1.09795D−01−4.55203D−04 −8.60311D−06 8.42391D−07 −8.03342D−08 14 1.99102D+010.00000D+00 0.00000D+00 −6.01215D−04 8.90015D−06 7.03373D−07−1.99661D−07 4.55392D−09

[0099] Numeric Example 3 f = 6.50 − 13.00 fno = F2.8 − 4.0 2ω = 63.2° −34.2° r1 = 66.701 d1 = 1.00 n1 = 1.49171 ν1 = 57.4 r2 = 6.338 d2 = 1.64r3 = 44.211 d3 = 0.80 n2 = 1.80400 ν2 = 46.5 r4 = 7.579 d4 = 0.89 r5 =9.062 d5 = 2.20 n3 = 1.58306 ν3 = 30.2 r6 = −52.336 d6 = Variable r7 =(Aperture stop) d7 = 0.00 r8 = 6.295 d8 = 2.20 n4 = 1.49171 ν4 = 57.4 r9= −22.489 d9 = 1.14 r10 = −10.437 d10 = 0.90 n5 = 1.58306 ν5 = 30.2 r11= 5.764 d11 = 0.65 r12 = 40.470 d12 = 0.50 n6 = 1.58306 ν6 = 30.2 r13 =8.079 d13 = 2.00 n7 = 1.80400 ν7 = 46.6 r14 = −11.487 d14 = Variable r15= 18.841 d15 = 1.60 n8 = 1.49171 ν8 = 57.4 r16 = −107.205 d16 = Variabler17 = ∞ d17 = 2.80 n9 = 1.51633 ν9 = 64.2 r18 = ∞ Focal length variableinterval  6.50  9.80 13.00 d6 11.16 6.39 1.93 d14 2.68 11.54 13.96 d165.11 1.16 2.01

[0100] Aspherical coefficient Surface number r K B C D E F G 26.33756D+00 0.00000D+00 −5.84197D−03 −6.49263D−04 −8.82664D−07−6.81704D−07 1.92883D−08 −3.72731D−10 8  6.29514D+600 0.00000D+001.81677D−02 −6.77949D−05 5.27603D−06 −4.38664D−08 −2.03598D−09 151.88409D+01 0.00000D+00 0.00000D+00 −2.77373D−04 2.57879D−05−1.85356D−06 4.94452D−08

[0101] TABLE 1 Lower Upper Numeric Numeric Numeric limit limit Example 1Example 2 Example 3 Conditional f2s 8.715 8.612 6.924 Expression (1) f212.820 10.662 10.021 f2s/f2 0.5 0.9 0.680 0.808 0.691 Conditional n2p1.49171 1.49171 1.49171 Expression (2) n2n 1.58306 1.58306 1.58306n2p/n2n 0.9 1 0.942 0.942 0.942

[0102] According to the zoom lens of each embodiment described above, itis possible to realize a zoom lens that is suitable for an image pickupsystem using a solid-state image pickup element, that is small in thenumber of lenses, that is compact, and that has a superior opticalperformance.

[0103] Next, a description will be given of an embodiment of an imagepickup apparatus (digital still camera) provided with theabove-mentioned zoom lens with reference to FIGS. 13(A) and 13(B).

[0104]FIG. 13(A) is a front view of a digital still camera, and FIG.13(B) is a side sectional view thereof. In the figures, 10 denotes acamera body (case), 11 denotes an image pickup optical system providedwith the zoom lens of any one of Numeric Examples 1 through 5, 12denotes a finder optical system, and 13 denotes a solid-state imagepickup element (photoelectric conversion element) such as a CCD or aCMOS sensor. The solid-state image pickup element 13 receives an imageof an object formed by the image pickup optical system 11 and convertsit to electrical information. Image information about the objectobtained by the conversion to electrical information is stored in astorage portion not shown.

[0105] A compact image pickup apparatus can be realized by applying thezoom lens of this embodiment to an image pickup optical system of adigital still camera in this way.

[0106] While preferred embodiments have been described, it is to beunderstood that modification and variation of the present invention maybe made without departing from the spirit or scope of the followingclaims.

What is claimed is:
 1. A zoom lens comprising, in order from an objectside to an image side, a first lens unit having negative optical power,the first lens unit including at least one positive lens and at leastone negative lens; a second lens unit having positive optical power, thesecond lens unit including at least one positive lens and at least onenegative lens, the lens of the second lens unit disposed on theclosest-to-object side being a plastic lens having positive opticalpower; and a third lens unit having positive optical power; wherein eachlens unit moves in the direction of an optical axis for zooming.
 2. Thezoom lens according to claim 1, wherein a surface on the object side ofsaid plastic lens of positive optical power is aspherical.
 3. The zoomlens according to claim 1, wherein the following condition is satisfied:0.5<f2s/f2<0.9 wherein f2s is a focal length of said plastic lens ofpositive optical power, and f2 is a focal length of said second lensunit.
 4. The zoom lens according to claim 1, wherein said second lensunit has a plastic lens with negative optical power, which is adjacentto an image side of said plastic lens of positive optical power.
 5. Thezoom lens according to claim 4, wherein the following condition issatisfied: 0.9<n2p/n2n<1.0 wherein n2p is a refractive index of materialof said plastic lens of positive optical power, and n2n is a refractiveindex of material of said plastic lens of negative optical power.
 6. Thezoom lens according to claim 1, wherein said third lens unit has aplastic lens with positive optical power.
 7. The zoom lens according toclaim 1, wherein each lens unit has at least one plastic lens, and thetotal number of plastic lenses included in each lens unit is even, andthe number of plastic lenses of positive optical power is equal to thenumber of plastic lenses of negative optical power.
 8. The zoom lensaccording to claim 1, wherein each lens unit has at least one plasticlens, and the plastic lens included in each lens unit is an asphericallens.
 9. The zoom lens according to claim 1, wherein said third lensunit moves to the object side for focusing from an infinite-distanceobject to a short-distance object.
 10. The zoom lens according to claim1, wherein said zoom lens forms an image on a photoelectric conversionelement.
 11. An image pickup apparatus comprising: the zoom lensaccording to claim 1, and a photoelectric conversion element by which animage formed by said zoom lens is received.